Typical ink jet printing processes which have heretofore been known include a process wherein ink is jetted as ink droplets through a fine nozzle. The nozzle typically has a diameter of several microns to several hundred microns. Each ink droplet is provided with a predetermined electrical charge according to an electric signal and then is subjected to electrostatic deflection by a high voltage deflection electrode. By controlling the deflections the ink forms letters, figures, etc. on the surface of a member to be printed, according to a predetermined dot matrix. According to another process ink is electrostatically withdrawn as ink droplets through a fine nozzle by the use of a high voltage electrode. Each droplet is accelerated and then subjected to electrostatic deflection with a deflection electrode, forming letters, figures, etc. on the surface of a member to be printed, according to a predetermined dot matrix. In still another process ink droplets are jetted in response to an electric signal, forming letters, figures, etc. on the surface of a member to be printed, according to a predetermined dot matrix.
Ink jet printing processes are basically grouped into two systems. In one system ink is jetted as ink droplets and charged, and the thus-charged ink droplet is electrostatically controlled, forming letters, figures, etc. on the surface of a member to be printed. In another system ink droplets are jetted through a fine nozzle in response to an electric signal, forming letters, figures, etc. on the surface of a member to be printed.
Aqueous inks useful in ink jet printing processes are comprised mainly of water and contain a hydrophilic organic solvent, a coloring dye and, if necessary, other various additives. These inks are useful for printing letters, figures and pictures on the surface of various members, such as plastics, metal, paper and cloth.
Aqueous inks must have the following characteristics:
(1) the ability to form uniform ink droplets when jetted through a fine nozzle:
(a) a viscosity of from about 1 to 10 cps (20.degree. C.); PA1 (b) a surface tension of about 20 dye/cm or more; and PA1 (c) a uniform quality containing no uneven particles. PA1 (a) have good drying properties, i.e., be capable of being rapidly dried after ink jet printing, so that letters, figures, and pictures formed are not deformed; PA1 (b) have sufficient water-resistance such that letters, figures and pictures formed do not run even though contacted by water; and PA1 (c) be capable of strongly adhering onto the surface of a member to be printed. PA1 (a) contain a dye having a color required for the final colored image; PA1 (b) contain a dye having as much yellow, magenta or cyan as possible when using a subtractive color process, and have an absorption band in a desired range with the other regions of the absorption band reduced as much as possible; PA1 (c) be capable of producing a colored image sufficiently sharp and free from staining; PA1 (d) contain a dye which exhibits high saturation and density when printed on a member, such as paper, and furthermore when stored as ink or even after it is printed, does not change with respect to color intensity, i.e., it has high light-resistance and is not subject to changes in color; PA1 (e) contain a dye having a high solubility in water so as to provide an ink having a high color intensity; PA1 (f) contain a dye which does not excessively increase the viscosity of ink; and PA1 (g) contain a dye of low crystallinity so that when stored or allowed to stand as an ink, it does not cause blocking of a nozzle by the crystallization or solidification thereof. PA1 C.I. Direct Yellow 27 (C.I. 13950) PA1 C.I. Direct Yellow 28 (C.I. 19555) PA1 C.I. Direct Yellow 33 (C.I. 29020) PA1 C.I. Direct Yellow 39 PA1 C.I. Direct Yellow 58 PA1 C.I. Direct Yellow 86 PA1 C.I. Direct Yellow 100 PA1 C.I. Direct Red 63 PA1 C.I. Direct Red 75 (C.I. 25380) PA1 C.I. Direct Red 79 (C.I. 29065) PA1 C.I. Direct Red 80 (C.I. 35780) PA1 C.I. Direct Red 83 (C.I. 29225) PA1 C.I. Direct Red 99 PA1 C.I. Direct Red 220 PA1 C.I. Direct Red 224 PA1 C.I. Direct Violet 47 (C.I. 25410) PA1 C.I. Direct Violet 48 (C.I. 29125) PA1 C.I. Direct Violet 51 (C.I. 27905) PA1 C.I. Direct Violet 90 PA1 C.I. Direct Violet 94 PA1 C.I. Direct Blue 1 (C.I. 24410) PA1 C.I. Direct Blue 8 PA1 C.I. Direct Blue 71 (C.I. 34140) PA1 C.I. Direct Blue 76 (C.I. 24411) PA1 C.I. Direct Blue 78 (C.I. 34200) PA1 C.I. Direct Blue 80 PA1 C.I. Direct Blue 86 (C.I. 74180) PA1 C.I. Direct Blue 90 PA1 C.I. Direct Blue 106 (C.I. 51300) PA1 C.I. Direct Blue 108 (C.I. 51320) PA1 C.I. Direct Blue 123 (C.I. 26705) PA1 C.I. Direct Blue 163 (C.I. 33560) PA1 C.I. Direct Blue 165 PA1 C.I. Direct Black 19 (C.I. 35255) PA1 C.I. Direct Black 38 (C.I. 30235) PA1 C.I. Direct Black 71 (C.I. 25040) PA1 C.I. Direct Black 74 (C.I. 34180) PA1 C.I. Direct Black 75 (C.I. 35870) PA1 C.I. Direct Black 112 PA1 C.I. Direct Black 117 PA1 C.I. Acid Yellow 17 (C.I. 18965) PA1 C.I. Acid Yellow 19 PA1 C.I. Acid Yellow 25 (C.I. 18835) PA1 C.I. Acid Yellow 29 (C.I. 18900) PA1 C.I. Acid Yellow 38 (C.I. 25135) PA1 C.I. Acid Yellow 49 PA1 C.I. Acid Yellow 59 PA1 C.I. Acid Yellow 61 PA1 C.I. Acid Yellow 72 PA1 C.I. Acid Red 1 (C.I. 18050) PA1 C.I. Acid Red 8 (C.I. 14900) PA1 C.I. Acid Red 32 (C.I. 17065) PA1 C.I. Acid Red 37 (C.I. 17045) PA1 C.I. Acid Red 42 (C.I. 17070) PA1 C.I. Acid Red 57 PA1 C.I. Acid Red 115 (C.I. 27200) PA1 C.I. Acid Red 119 PA1 C.I. Acid Red 131 PA1 C.I. Acid Red 133 (C.I. 17995) PA1 C.I. Acid Red 134 (C.I. 24810) PA1 C.I. Acid Red 154 (C.I. 24800) PA1 C.I. Acid Red 186 (C.I. 18810) PA1 C.I. Acid Red 249 (C.I. 18134) PA1 C.I. Acid Red 254 PA1 C.I. Acid Red 256 PA1 C.I. Acid Violet 11 (C.I. 17060) PA1 C.I. Acid Violet 34 (C.I. 61710, 61800) PA1 C.I. Acid Violet 26 PA1 C.I. Acid Violet 75 PA1 C.I. Acid Blue 29 (C.I. 20460) PA1 C.I. Acid Blue 126 PA1 C.I. Acid Blue 171 PA1 C.I. Acid Blue 175 PA1 C.I. Acid Blue 183 PA1 C.I. Acid Black 1 (C.I. 20470) PA1 C.I. Acid Black 24 (C.I. 26370) PA1 C.I. Acid Black 26 (C.I. 27070) PA1 C.I. Acid Black 48 (C.I. 65005) PA1 C.I. Acid Black 52 (C.I. 15711) PA1 C.I. Acid Black 58 PA1 C.I. Acid Black 60 PA1 C.I. Acid Black 107 PA1 C.I. Acid Black 109 PA1 C.I. Acid Black 119 PA1 C.I. Acid Black 131 PA1 C.I. Acid Black 155 PA1 C.I. Reactive Blue 7 PA1 C.I. Reactive Blue 14 PA1 C.I. Reactive Blue 15 PA1 C.I. Reactive Blue 18 PA1 C.I. Reactive Blue 21 PA1 C.I. Reactive Blue 25 PA1 C.I. Reactive Yellow 1 PA1 C.I. Reactive Yellow 2 PA1 C.I. Reactive Yellow 3 PA1 C.I. Reactive Yellow 13 PA1 C.I. Reactive Yellow 14 PA1 C.I. Reactive Yellow 15 PA1 C.I. Reactive Yellow 17 PA1 C.I. Reactive Red 2 PA1 C.I. Reactive Red 6 PA1 C.I. Reactive Red 11 PA1 C.I. Reactive Red 23 PA1 C.I. Reactive Red 36 PA1 C.I. Reactive Violet 2 PA1 C.I. Reactive Violet 4 PA1 C.I. Reactive Violet 8 PA1 C.I. Reactive Violet 9 PA1 C.I. Basic Yellow 11 PA1 C.I. Basic Yellow 14 PA1 C.I. Basic Yellow 21 PA1 C.I. Basic Yellow 32 PA1 C.I. Basic Red 1 PA1 C.I. Basic Red 2 PA1 C.I. Basic Red 9 PA1 C.I. Basic Red 12 PA1 C.I. Basic Red 13 PA1 C.I. Basic Violet 3 PA1 C.I. Basic Violet 7 PA1 C.I. Basic Violet 14 PA1 C.I. Basic Blue 3 PA1 C.I. Basic Blue 9 PA1 C.I. Basic Blue 24 PA1 C.I. Basic Blue 25
(2) In order to form clear, sharp letters, figures, etc. on the surface of a member to be printed the ink must:
(3) In order to obtain good colored images on the surface of a member to be printed the ink must:
It is known that a humectant is incorporated into ink in order to meet the foregoing requirements. In particular, this minimizes the undesirable drying properties of ink at a nozzle. Examples of such humectants known in the art include alkylene glycols and alkylene glycol alkyl ethers, wherein the alkylene group contains 2 to 6 carbon atoms and the alkyl group contains 1 to 6 carbon atoms, as described in U.S. Pat. Nos. 3,705,043 and 3,846,141; water-soluble organic solvents, such as formamide and dimethylformamide, as described in Japanese Patent Publication No. 6881/1978; N-hydroxyalkylformamides as described in Japanese patent application (OPI) No. 97620/1974 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"); and N-hydroxyalkyl-2-pyrrolidones as described in Japanese Patent Publication No. 16243/1979.
It is also known that for the purposes of obtaining high saturation and preventing aggregation and a change in color intensity of dye with a lapse of time, a solubilizing agent is added to ink in combination with a humectant. Such solubilizing agents include N-methyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, substituted-pyrrolidones, 4-methoxy-4-methylpentanone-2, and tetrahydrofurfurylalcohol, which are described in U.S. Pat. No. 3,846,141.
The foregoing alkylene glycol-based solvent generally has a high viscosity and a low vapor pressure, and therefore it can prevent, to a certain extent, the drying or solidification of an aqueous ink when the jet flow of aqueous ink is stopped. However, under extreme conditions wherein the aqueous ink is jetted for a long period of time, interrupted for about one week, and again jetted, the dye or solids comprising the dye and ink additives precipitate in the nozzle. This causes blocking of the nozzle and inhibits smooth jetting of the ink. By increasing the amountof the alkylene glycol-based solvent added, the drying and solidification of the ink remaining in the vicinity of the nozzle can be improved to a certain extent, but cannot be substantially improved. An increase in this solvent leads to an increase in the viscosity of ink. Accordingly, the ink cannot flow through the nozzle at a sufficient velocity, resulting in the occurrence of a disarray of printed images.
The alkylene glycol alkyl ether-based solvent generally has a low surface tension. Therefore, when an ink droplet containing such an alkylene glycol alkyl ether-based solvent is attached onto the surface of a member to be printed, it is likely to run, and with members such as paper and cloth, it penetrates from the top surface to the back surface and runs on the surface, making the final image blurred. Furthermore, the alkylene glycol alkyl ether-based solvent is disadvantageous due to its low solubility in an aqueous dye, so that it cannot provide a sufficiently colored image. Furthermore, it increases the viscosity of ink.
The amide-based solvent is significantly hydrolyzed in ink, i.e., an aqueous solution, resulting in a decrease in the pH. Therefore, it cannot be used in those dyes which are greatly dependent on pH. This limits the range of water-soluble dyes which can be used. Because of its high corrosive action on metal and plastics, it is necessary to use a specific ink jet printing apparatus. The amide-based solvent is effective in preventing the blocking of the nozzle unit to a certain extent. However, this anti-clogging effect is not satisfactory. Furthermore, it fails to provide a color image having sufficient saturation and color density.